1991 RS 305

OK, I KNOW this topic has been beaten to death, but heres my problem with a 52mm being "too big". We do all these entry level mods like mine below for every molecule of breathing available. How can going from a 48mm to a 52mm , which would make more air available, espeically at WOT, possibly make you go SLOWER? Could someone explain this concept to me? Why Supercharge or Turbocharge, then? I have been led to believe MORE AIR is better, not just a little more. I would think the velocity would be AT LEAST as much with a 52mm, since the air will flow into the engine as fast as it can be drawn in, and more would be available. The base/heads will be the limiting factor, so the MAX amount of air would be available despite these limitations. Make sense? Comments?

unvc92camarors

i'm no physics teacher but...i think this applies (maybe)
think of a vacuum..like shopvac or ridgid or something like that
on the hose you have a tip, and if its the right size, you get a good amount of air flowing through it at a high velocity
but if you put too big of a tip on (too wide, too tall, whatever) it flows really really slow, thus not letting as much air get in and decreasing velocity immensely

jfreeman74

I might could see this on a stock setup but if you go to an aftermarket heads, cam, bigger intake manifold I would think that the bigger TB could only help.

RedIrocZ-28

Approaching 400hp is where a twin 48mm TB becomes restrictive.

A twin 52mm TB reduces velocity because of the cross sectional area for air induction has become significantly larger....

Velocity is good, thats why the LS1 pulls hard down low and up top. Take a stock LS1 and put a 58mmTB and that probably won't be the case anymore.

1991 RS 305

Another reason for the question is because the current (August) issue of HOT ROD is doing a series of bolt on tests with dyno runs after each mod for a (gasp) 2001 4.6 Mustang. Page 52 shows a throttle body swap from the OEM, which looks almost exactly like a TPI throttle body, to a "Accufab" one piece eliptical butterfly. The divider had to be removed from the plenum opening in order to use the new part. The "mm" or CFM size is not mentioned for either throttle body, but its clear the "Accufab" is huge. The engine is otherwise stock up to this point except for bolt-ons: headers, pulleys, Hypertech reprogram. The larger throttle body, according to this test, added power below 3500 with gains of 10-11 hp below peak, and 5 hp and 7 ft lbs at peak , and helped the engine run up to 5700 instead of laying down at 5300. Now, I know these tests can be questionable, but heres my point: if more air apparently helps a stock 4.6 breathe deeper and make more power, why not a 5.0 305?

Homer

iTrader: (1)

Too large a throttle body cannot be a bad thing. It doesn't matter what the velocity through the throttle body is. There's no fuel to mix up there or anything. By the time it goes through a big plenum, down some runner, to the port, whatever velocity you had at the throttle body is irrelevant.
The only problem I see is this. Does the computer know you're using a bigger throttle body? It has a throttle position sensor that it translates into how much air it's getting. 25% open on a 52 mm throttle body is going to give you more air than 25% open on a 48 mm throttle body.

Dano 00TA

Unless you have a pretty stout 305 it just aint worth it. As the old saying goes, bigger isn't always better. Just like a carb, you put a 850dp on a relativly stock 350 and it'll be a pig on the bottom end. I guarantee that a 600vacum secondary will have alot more bottom end grunt than the 850. Sure the 850 might even the score or beat it hp wise on the top end but by that time you've already had your a$$ handed to you by the guy/girl that spent their money elsewhere instead on the biggest carb tb they could find

Same goes with the exhaust. Ever notice with open headers or an open cutout it feels like it lost some low end?

Homer

iTrader: (1)

The carb needs velocity to properly meter and atomize the fuel. The throttle body doesn't do any of this. You can't compare a throttle body to a carburetor. The carburetor needs to be properly sized to the application. The throttle body can be as big as you want. Think of it this way: have you ever heard someone say an air filter is too big? Or doesn't provide enough velocity, or backpressure? No, because that's hogwash. Throttle body is the same way.

That's the theory. In practice, your stock unit is just fine.

jfreeman74

Dano, going by what you're saying it would make sense that going to a bigger intake manifold and runners would be an even bigger waste of money. All this may be true for a stock setup but if you put in new heads, cam, and exhaust, it would stand to reason that you need more air coming in. You're not going to get the most out of heads and cam without doing the intake.

Dano 00TA

Your right if you go to a good flowing heads and a good sized cam....and plan to run some rpms outta her. Yes, you do need to upgrade your induction but I'm saying you aint gonna pick up any power unless the engine needs it. Like I said if it's built pretty stout then yes if it's not then not yet. Your money can be better spent on something to give you power

jfreeman74

Ahh, okay I got you. I do agree with you about that. If it is a stock 305 then I would not recommend the TB either. I have, IMO, a very stout 305. I am going to add heads and a cam soon so that is why I am looking to get a bigger TB and intake.

89transam

Why waste the money despite many knowledgeable members insisting (with proof) that is worthless? Do you think that your car will somehow react diffrently.

Also how do you gain 30 HP and as a result drop 1.6 seconds on your 1/4? (Your sig)

Morley

A bigger cam and set of world products heads are not going to require the additional air that a larger TB can flow.
The 305 needs a lot more than just those 2 components to make anywhere near 400 hp.
If you went to a set of AFR or TFS heads, Bigmouth manifold, siameased runners, ported plenum, 1.75" headers, higher compression, agressive cam, then you may need the extra air provided by a larger TB, but until then, it is just wasted money and possibly a weaker engine.
But, its your money to spend however you want.

I always take the "results" from a magazine's project build ups with a bag of salt (not grain, bag), they have a vested interest in showing that what they are doing is maknig power. If it showed a power loss or no gains at all, would you then run out and buy the products they tested?

jfreeman74

Those are old HP numbers. I'm not sure what they are right now. I plan on going to the dyno soon. I want to get it dynoed before I put the heads and cam in.

Blacker

Torque Converters do not show gains on a dyno, they show the gains at the track. With my Yank TT3000, the dyno gains were less than 1rwhp, but gained 6 tenths at the track. Add Nitto Drag Radials, my gains were 0.98seconds with no gains on the dyno. Anyway, back to the scheduled programming

bigals87z28

not worth it... unless your forcing some fresh air, and have heads, cam, etc to back it up... its a waste.. $300 could be spent better IMHO. suspension, cam, ported heads, intake manifold, tools, TC, aluminum drive shaft, gears, 4th gen rear, a hell of a night on the town with some hot *** chick!! you get the drift man...

rx7speed

actually I have seen someone do some test with air filters

to be perfectly honest the larger airfilters sucked (loss of at most 10hp or so)

while the smal 4 inch rinky dink things worked best

but this was setup on a carb

1991 RS 305

I see lots of people have read this, it is interresting. Now, if I'm understanding this right, on a stock 85 305, which has the "L69" cam, NOT the peanut cam(86 up), the 48mm throttle body is flowing AT LEAST as much, if not more air, than the heads, base, or runners can flow? In other words, the restrictions probably are, in this order:

1) The base
2) The runners
3) The heads

If a person had a line on a good 52mm throttle body for cheap, isn't there at least the "possibility" that additional available air would maximize everything downstream from it? I still don't see how letting in more air at the point of entry can be anything but good. Any more comments?

89transam

read this. Made me not buy a 52 mm tb

https://www.thirdgen.org/techbb2/sho...hreadid=104116

25THRSS

I doubt it. You can't go too big on an air filter. All it does is filter air and the more filtering the better. You want the air filter to not restrict any air going into the engine, yet smooth it out at the same time. This is why you might actually see a loss by taking the filter completely off. Incase anyone cares, here is a cool study on the different brand air filters out there. Notice how the most restrictive filters have more swirl to them.

http://www.fordmuscle.com/archives/2...s/index2.shtml

SLP IROC-Z

iTrader: (1)

this thread is full of misleading facts, the air cleaner one just took the cake. looks like uve dynoed ur car a few times, why not buy a 52mm and maybe even a 58mm while ur at it and dyno all three back to back the 48, 52 and 58 and get back to us on what does what.

CrazyHawaiian

If the TPI intake is stock then the throttlebody wont give you much more performance because the plennum, runners, and base would still be big bottlenecks. What it comes down to is, no single mod like this will make a big difference. Its a combo of mods that make a big difference. And remember this includes a re-tune. If you want more flow then you need to mod the entire intake, not just the throttlebody. You can do just the throttlebody but dont expect a big difference. Consider it one part of a long list of mods you'll need to feel a difference. Take my 91Z for example, I was doing my thermostat one time and I remembered I had a BBK twin 58mm TB in my parts stash (got it for free when I bought the L98 for my 89 RS). I installed the bigger TB, but the rest of my TPI intake was still stock. The only difference I saw was better throttle response, but even that was minimal. I did not gain any real performance over the stock twin 48mm with the airfoil, and my motor is a supercharged L98. Just as I expected. I'm not going to gain alot of power untill I do the whole intake and get the ECM retuned for the new airflow. But to answer your question, no I dont think doing the TB alone will decrease performance at all. It didnt happen to me ...

Fevre

Might want to read this:



Make sure to read the max tq for each. Although not a clear cut case for using a smaller filter, it does seem to indicate that filter selection should not be based on the biggest you find.

SLP IROC-Z

iTrader: (1)

his car is not carburated therefore that article is not relivent to this thread.

Morley

How so?

rx7speed

dont understand why not?

both styles have a filter sitting on the end of a tube sucking air in


just because one is injected with fuel makes the whole difference?

Ed Maher

OK, so what's your point. I think the most important part of that article is:


So there it is, in black and white, in your own source. There is NO SUCH THING AS TOO BIG OF A FILTER.* If all the #s are within error tolerance, then they are identical, period. And even Marlan says that the small filters show evidence of restriction in plain writing. So i really don't get your point.


*Note, there are some transient issues with too big of a filter, but it is strictly a tuning issue. With a larger filter, or more correctly a larger effective plenum on top of the throttle blades, you will tend to need more pumpshot since that mass of air within the filter accelerates into the engine more quickly than if it had to come from behind the filter. BTW, this phenomena is also evidence of a stronger initial reaction with a larger filter as well.

Fevre

If spending a sh it load of $ on the largest filter you can fit/find makes you think you will make more power than go ahead. A filter can be too big: A filter is too big if it costs you more money and doesn't make any more power, sorta like a 52mm tb on a 305. Now if the 305 is super warmed and you need the 52mm tb on it then by all means do, of course that doesn't mean it needs a 58mm tb justs because it fits and flows more air.